Add Tomb support

Add Tomb support to prs, similar to what pass-tomb provides.

Suggested in: https://github.com/timvisee/prs/issues/8

With pass-tomb some manual actions are required to open and close the tomb. Within prs I'd like to automate as much as possible to ease things for the end user. pass-tomb can already be used with prs as prs is compatible with pass, but this still requires the manual invocations.

Implementation ideas:

• Create tomb for existing store with prs tomb init.
• If store uses tomb: automatically open on use, keep open for ~1 minute to keep subsequent commands fast.
• Automatically grow tomb if filesystem becomes too small (see tomb resize).
• Decide what to do with the 'no swap' check (see tomb -f).
• Support same tomb/tomb key paths as pass-tomb uses for compatibility.

Useful bits:

• One-shot systemd service to close tomb after some delay: https://github.com/roddhjav/pass-tomb/blob/master/pass-close%40.service
• Tomb with GPG keys: https://pujol.io/blog/tomb-with-gpg-keys/

TODO:

• Implement tomb interface in lib
• Make password store Tomb-aware
• Implement open/close command
• Implement init command
• Implement status command
• Implement resize command
• Set up timer to automatically close Tomb
  • Force create timer even if it already exists (might happen in some weird situations)
• Update README:
  • Features list
  • Requirements
  • Compiler features
  • Write about Tomb support in security section
• Decide what to do with no-swap -f flag
• Implement automatic resize if Tomb becomes too small
• Mark tomb as dependency in relevant packages
• Update feature flag in relevant package builds
• Fix GTK3 copy widget not prompting for Tomb password
• Fix pinentry prompt on Wayland, set GPG_TTY for it